For pressurized flow, I would try to find a relatively cheap program that includes a water quality module. 20 years ago, Flownet worked great. It may still be available. Most of them just calculate the average residence time based on average flow demand. There are some that are completely dynamic. I used it to determine areas with dead ends in long runs of water lines without sufficient demand in order to determine whether we could remove those dead ends through system expansion, or we needed to continue to flush.
It gets a little more complicated for non-pressurized systems. If you want a very accurate answer, then you need to pull out partially full pipe equations, based on flow rates and Manning's Equations. V. T. Chou has a great section in his red book. (A must have for most water resource engineers.) Most jurisdictions allow you to calculate the TC by the travel time, without considering whether the pipe is partially full. Velocity is Flow / Area of Culvert. Travel time = length / velocity. Make sure you keep your units consistent. Every jurisdiction has their own rules, so make certain you get a good look at their design criteria prior to submitting things. The travel time in the ditch is probably going to be much more significant for most projects unless this is a very long culvert.
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Dwayne Culp, Ph.D., Ph.D.,M.ASCE
Culp Engineering, LLC
Rosenberg TX
(713)898-1977
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Original Message:
Sent: 04-24-2019 17:07
From: Ray Francis Ralota
Subject: Time Concentration
Hello fellow Civil Engineers,
I am Ray, a newly practicing civil engineer in the field of Water Resources Engineering. I would like to ask for your advise about what would be the best empirical formula to use in determining the time concentration of culvert pipes?
PS: Tried searching for the same topic here but got none from the search box.
Best Regards,
Ray
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Ray Francis Ralota R.Eng,Aff.M.ASCE
Civil Engineer
Cebu
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